1. Field of the Invention
The present invention relates generally to an electrical circuit for synchronizing and phase shifting two oscillators, for example, oscillators used in pulse width modulation circuits.
2. Description of the Related Art
Electronic devices, such as computers, use a power supply unit to supply power to other internal components of the device. The different components often require different supply voltages which generally must be generated from a single voltage of the power supply unit. The voltage supplied by the power supply unit of the electronic device is often a DC voltage such as 12 volts or 5 volts and this DC voltage must be converted to other DC voltages which may be either higher or lower for use by certain of the components. In one example, the voltage of the power supply unit is to be converted to 3.3 volts and 2.5 volts for components in the device. It is also necessary to provide multiple voltage converters when multiple high current components, such as multiple processors, are used.
One approach to converting a DC voltage to a second DC voltage makes use of pulse width modulation (PWM). In pulse width modulation, the input voltage is turned on and off by a repetitive pulse having a period proportionate to the reduction in voltage desired. The pulsed signal is forwarded to an averager to achieve the output voltage. For example, a 5 volt DC level may be converted to a 2.5 volt level by using a pulse width that is on 50% of the time and off 50% of the time and which is averaged to achieve the 2.5 volt level. A higher voltage may be achieved by having a proportionately higher on time for the pulsed signal, and conversely, a lower voltage may be achieved by a shorter on time for the pulsed signal.
An oscillator is used to generate the periodic signal in a pulse width modulator circuit, the PWM being controlled by a feedback circuit that adjusts the pulse width to produce the desired output level.
Where two or more output voltage levels are required, two pulse width modulation circuits may be utilized. When using multiple pulse width modulation controllers in a common switch mode power supply, it is desirable to synchronize the individual oscillators to a common switching frequency. This helps prevent electrical noise generated by sum and difference beat frequencies. In addition, the oscillators should be phase shifted apart to prevent large input current pulses caused by simultaneous turn-on of each power pulse.
An example of a circuit for synchronizing two oscillators is illustrated in FIG. 1. A first oscillator 10 is synchronized and phase locked to a second oscillator 12 by a synchronizing circuit utilizing an integrated circuit chip 14 and a variety of components including precision capacitors and resistors. In particular, the oscillator 10 begins a switching cycle and in doing so emits a signal from an output on pin 12 that is connected to a gate of a FET 25. The signal passes through a resistor 20 and between schottky diodes 26 and 27 to an input pin 2 of the chip 14. This triggers a one-shot in the chip 14. The one shot has been set, using precision capacitors and the like, to generate a pulse of a length of approximately one half of the switching period of the oscillator 10. The falling edge of the pulse triggers a second one-shot in the chip 14, which produces a very short pulse. This short pulse is connected to the timing ramp input of the slave oscillator 12 through capacitor 16 and 18 and resistors 17 and 19. The second oscillator 12, upon receiving the short pulse, starts a new switching cycle, which outputs a pulse on its pin 12 to the gate of a FET 28. The pulse lengths of the two one-shots in the chip 14 are set by the capacitor 22 and 23 and the resistors 21 and 24.
An object of the present invention is to synchronize two oscillators together and to offset, or phase shift, the timing signals of the oscillators from one another.
A further objective of the present invention is to provide a synchronization and phase shift circuit for oscillators using only a few non-precision parts.
The present invention achieves these and other objectives by providing a filter to turn a sawtooth signal output from a first oscillator into a DC voltage that is used to identify a phase shift point for the second oscillator. The present circuit utilizes a comparator with feedback hysteresis, a filter, and a synchronization injector to link the master oscillator output to the slave oscillator input.
The present invention achieves a synchronization between the pulses of the master and slave oscillators and introduces a phase shift, such as 180xc2x0, between these pulses to avoid current surges resulting from simultaneous firing of the two oscillators. The present circuit does not require the use of precision components and uses fewer components and lower cost components than prior circuits.